1. The Field of the Invention
The present invention relates generally to dilatation balloon catheters and systems used for expansion against an obstruction within a body vessel or channel, or to deliver devices such as, but not limited to, stents and therapeutic agents to sites within vascular or tubular channel systems of the body.
2. The Relevant Technology
The present invention relates to dilation balloon catheters employed applications such as percutaneous transluminal angioplasty (PTA) and percutaneous transluminal coronary angioplasty (PTCA) procedures, and more particularly to enhancements to such catheters and their dilatation balloons for improved maneuverability in smaller and more tortuous passages of the vascular system.
Dilatation balloon catheters are well known for their utility in treating the build-up of plaque and other occlusions in blood vessels. Typically a catheter is used to carry a dilatation balloon to a treatment site, where fluid under pressure is supplied to the balloon, to expand the balloon against an obstruction. Additionally, the expansion of the balloon may deploy a stent device in the treatment area.
The dilatation balloon usually is mounted along the distal end region of the catheter and surrounds the catheter. When the dilatation balloon is expanded, its main body portion or medial section has a diameter substantially larger than that of the catheter. Proximal and distal shafts or stems of the balloon have diameters substantially equal to the diameter of the catheter. Proximal and distal tapered sections, or cones, join the medial region to the proximal and distal shafts, respectively. Each cone diverges in the direction toward the medial region. Bonds between the balloon and catheter form a fluid tight seal to facilitate dilatation of the balloon by introduction of a fluid under pressure.
Along with body tissue compatibility, primary attributes considered in the design and fabrication of dilatation balloons are strength and pliability. A higher hoop strength or burst pressure generally reduces the risk of accidental rupture of a balloon during dilatation, although this is also dependent on the characteristics of the vessel lesion.
Pliability refers to formability into different shapes, rather than elasticity. In particular, when the balloon is an uninflated or deflated configuration, the dilatation balloon is evacuated, flattened and generally wrapped circumferentially about the catheter distal region. Thin, pliable dilatation balloon walls facilitate a tighter wrap that minimizes the combined diameter of the catheter and balloon during delivery. Furthermore, pliable balloon walls enhance the catheter “trackability” in the distal region, i.e. the capability to bend in conforming to the curvature in vascular passages.
One method of forming a strong and pliable dilatation balloon of polyethyleneterephthalate (PET) is disclosed in U.S. Pat. No. Re. 33,561 to Levy. A tubing of PET is heated at least to its second order transition temperature, and then drawn to at least triple its original length to axially orient the tubing. The axially orientated tubing is then radially expanded within a cylindrical form, to a diameter at least triple the original diameter of the tubing. The form defines the aforementioned main body, shafts and cones, and the resulting balloon has a burst pressure of greater than 200 psi.
Such balloons generally have a gradient in wall thickness along the cones. In particular, dilatation balloons with an expansion diameter in the range of 3.0-4.0 mm tend to have a wall thickness along the main body in the range of 0.0004-0.0008 inches (0.010-0.020 mm). Near the main body, the cones have approximately the same wall thickness. However, the wall thickness diverges in the direction away from the main body, until the wall thickness near each shaft is in the range of 0.001-0.0025 inches (0.025-0.063 mm). Smaller dilatation balloons (1.5-2.5 mm) exhibit the same divergence in the cone walls, i.e. from 0.0004-0.0008 inches near the main body to 0.0008-0.0015 inches (0.02-0.04 mm) near the associated shaft or stem.
The increased wall thickness near the stems does not contribute to balloon hoop strength, which is determined by the wall thickness along the balloon medial region. Thicker walls near the stems reduce maneuverability of the balloon and catheter. The dilatation balloon cannot be as tightly wrapped, meaning its delivery profile is larger, limiting the capacity of the catheter and balloon for treating occlusions in smaller vessels.
U.S. Pat. No. 4,963,133 to Noddin discloses an alternative approach to forming a PET dilatation balloon, in which a length of PET tubing is heated locally at opposite ends and subjected to axial drawing, to form two “necked down” portions which eventually become the opposite ends of the completed balloon. The necked down tubing is simultaneously axially drawn and radially expanded with a gas. The degree to which the tubing ends are necked down is said to provide control over the ultimate wall thickness along the tapered walls (or cones), so that the wall thickness can be equal to or less than the wall thickness along the main body. This approach, however, is said to result in a comparatively low burst pressure, only about 8 atmospheres, or about 118 psi.
Typically PCTA catheters can be classified as either having a compliant, semi-compliant or a non-compliant balloon. Compliance is defined as the increase in diameter from nominal balloon pressure to rated burst pressure. Non-compliant balloons have less than 7% increase in diameter. Semi-compliant balloon have between 7-10% increase in diameter and compliant balloons have at least 10% increase in diameter.
Typically a non-compliant balloon catheter is utilized first during most procedures and for very severe lesions. The non-compliant balloon catheter is used with high inflation pressure to crack calcified lesions. Subsequent to use of the non-compliant balloon catheter, a semi-compliant or a compliant balloon catheter may replace the non-compliant catheter for further vessel modeling.
Generally, the non-compliant balloon is used initially because of the balloons resistance to diameter growth, that is, very high pressure can be applied to the balloon without producing a significant diameter increase. A disadvantage of non-compliant balloon catheters is their inability to upsize a vessel. Upsizing is when a balloon is inflated at greater pressures causing the diameter to increase, for non-compliant balloons, upsizing is not possible because they are designed not to expand to a larger diameter at higher pressures. Thus, if the vessel needs to be upsized, the non-compliant balloon catheter must be removed from the vessel and a semi-compliant or compliant balloon catheter is then placed into the vessel.
Therefore there is a need for a balloon catheter having compliant, semi-compliant and non compliant properties.